1. Field of the Invention
The present invention relates to the field of liquid crystal displaying techniques, and in particular to an asymmetric prism structure, a light guide plate, a backlight module, and a related application.
2. The Related Arts
The so-called shutter glasses 3D display technique is the most popular solution by recent 3D LCD TVs. This technique displays images respectively for the left and right eyes by partitioning backlight blinking. Then, with synchronously blinking glasses, the left and right eyes perceive different images, thereby achieving stereoscopic visual effect. More specifically, this technique involves delivering the frame signals for the left and right eyes alternately to the LCD panel, driving the LCD panel to display the images for the left and right eyes, and, together with the scanning of the backlight module and the time-synchronized shutter glasses, making the viewer to perceive the images for the left and right eyes as a single 3D image.
The 3D LCD display has a disadvantage. Since the LCD panel requires a backlight module to provide illumination, the partitioning of the backlight cannot be too fine. FIG. 1 is a schematic diagram showing the partitioned illumination and leakage of an edge-lit LED backlight. The edge-lit LED backlight is to arrange LED dies along the circumference of a LCD panel. Then the light emitted from the edge of the LCD panel is delivered to the center of the LCD panel through a light guide plate so as to provide the required illumination to present the image on the LCD panel. The edge-lit LED backlight has two advantages. One is that fewer LED dies are required and as such cost is reduced. The other one is that the thickness of the LCD panel can be reduced as the LED module is at the side, not in the back.
As illustrated in FIG. 1, a backlight partition 11 is lit from the right side of the LCD panel. When the backlight partition 11 is lit, the light leaks into partitions 12 and 13, and, as the light travels farther, the leakage is more serious. The leakage would cause interference between the signals for the left and right eyes. In other words, the left eye would perceive the signal for the right eye or vice versa. The interference results in a blurred image as the two signals are distributed spatially apart. The degree of blur is measured by cross-talk. A greater cross-talk means a greater interference between the left- and right-eye signals. Therefore, a major R&D topic is to reduce cross-talk so as to maintain product competitiveness.
The problem of cross-talk between the left- and right-eye signals is inherent in the shutter glasses 3D display technique. According to the shutter glasses 3D display technique, the backlight module is separated vertically into an even number of backlight partitions. The time and duration of illuminating each backlight partition is controlled in accordance with the top-down image scanning. The image signal (for left or right eye) provides the driving voltage sequentially from top to bottom to the rows of pixels of the LCD panel. Under the charge of the driving voltage, the pixels of the LCD panel start to respond. Due to the design of the pixel and the viscosity of liquid crystal, a period of response time is required before the liquid crystal reach a steady state. Due to the required response time of the liquid crystal, images are scanned onto the LCD panel also by partitions. When the image signal for a partition of the LCD panel is scanned, a corresponding backlight partition is lit while the other backlight partitions are turned off. Due to the leakage described above, when the light of a backlight partition for a left-eye signal leaks to an adjacent backlight partition for a right-eye signal (or vice versa), the eye would perceive both left- and right-eye images (i.e., the cross-talk). The left- or right-eye signal causing the cross-talk is referred as error signal (or cross-talk signal).
FIGS. 2A and 2B are schematic diagrams showing the illumination of the backlight partitions of an existing 46-inch single-shorter-edge-lit LED TV. The backlight module 20 is usually separated into an even-numbered (e.g., 4) backlight partitions. When a topmost backlight partition 21 is lit, the backlight leaks to a lower backlight partition. When a middle backlight partition 22 is lit, the backlight leaks to both an upper backlight partition and a lower backlight partition.
FIG. 3 is a schematic diagram showing the measurement of cross-talk at 9 points on a LCD panel. As illustrated, the horizontal and vertical dimensions of the LCD panel 30 are denoted as H and V, respectively. Using an existing 46-inch single-shorter-edge-lit LED TV as example, the 9 points' cross-talk is measured and summarized in Table 1. As can be seen from Table 1, the cross-talk is not vertically symmetric with a greater value at upper points and a smaller value at lower points. The cross-talk is also not horizontally symmetric. This is due to light is incident from a side and, as it travels farther, the leakage is more serious.
TABLE 1Cross-talk at 9 points (46-inch, single-shorter-edge-lit, and4 backlight partitions).Single-edge-litLeft 1/9Middle 1/2Right 8/9Upper 1/914.99%8.84%7.03%Middle 1/25.60%4.51%3.69%Lower 8/98.47%6.20%4.81%
Due to the backlight partitions, signals for the LCD panel, glasses, and the backlight module have to be time-synchronized and this often leads to asymmetric cross-talk. From the data of Table 1, for an existing 46-inch, single-shorter-edge-lit LED TV, the left- or right-eye signal has the best image quality at the center of the LCD panel and the image quality is vertically asymmetric. The vertically asymmetric cross-talk shown in Table 1 can be explained by the time sequence relationship between signals to the backlight partitions and the LCD panel. FIG. 4 is a schematic diagram showing the time-sequence relationship between signal to the backlight partitions and signal to the LCD panel (i.e., the left- or right-eye image signal to the LCD panel) of an existing 46-inch, single-shorter-edge-lit LED TV (left-eye signal). The backlight module is vertically and sequentially separated into a first backlight partition 41, a second backlight partition 42, a third backlight partition 43, and a fourth backlight partition 44, for illuminating a first display partition, a second display partition, a third display partition, and a fourth display partition of a LCD panel 40, respectively. Using the left-eye signal as example, FIG. 4 shows four consecutive steps of the LCD panel 40 and the backlight partitions illuminating the LCD panel 40. In step a, the left-eye signal from a current frame is loaded into the first to third display partitions whereas the right-eye signal from a previous frame is loaded into the fourth display partition. The first backlight partition 41 is lit to illuminate the first display partition. Since the light from the first backlight partition 41 may leak to the fourth display partition, the right-eye signal from a previous frame loaded into the fourth display partition becomes the error signal causing cross-talk with the left-eye signal from a current frame loaded into the first display partition. As the first and fourth display partitions are separated by two display partitions in between, the cross-talk is mild. In step b, the left-eye signal from a current frame is loaded into the fourth display partition and, therefore, the complete left-eye signal for the current frame is loaded into the LCD panel 40. The second backlight partition 42 is lit to illuminate the second display partition and the leakage from the second backlight partition 42 does not cause any cross-talk. As such, the image quality is the best. In step c, the right-eye signal from a next frame is loaded into the first display partition whereas the left-eye signal from a current frame is loaded into the second to fourth display partitions. The third backlight partition 43 is lit to illuminate the third display partition. The right-eye signal from a next frame loaded into the first display partition becomes the error signal causing cross-talk with the left-eye signal from a current frame loaded into the third display partition. Since the first and third display partitions are separated by a display partition in between, the cross-talk is more serious as their distance is closer. In step d, the right-eye signal from a next frame is loaded into the first and second display partitions whereas the left-eye signal from the current frame is loaded into the third and fourth display partitions. The fourth backlight partition 44 is lit to illuminate the fourth display partition. The right-eye signal from a next frame loaded into the first and second display partitions becomes the error signal causing cross-talk with the left-eye signal from the current frame loaded into the fourth display partition. Since the first and second display partitions are separated from the fourth display partition by a display partition in between, the cross-talk is more serious as their distance is closer. In the entire 3D display process, the LCD panel 40 is loaded repeatedly with the right-eye signal (previous frame), the left-eye signal (the current frame), the right-eye signal (the next frame), the left-eye signal, the right-eye signal, etc. Since the existing edge-lit backlight modules are divided into an even-numbered backlight partitions. The error signal has different impact to those display partitions above and below. In the above example, the error signal where the lit time of the backlight partition closer to the top produces greater cross-talk in the top portion of the LCD panel 40. The cross-talk of the LCD panel 40 is therefore vertically asymmetric. If the signal to the LCD panel 40 is adjusted so that the backlight partition is lit in the middle of the signal to the LCD panel, the cross-talk would become more vertically symmetric. Yet, as the number of backlight partitions is even, the image quality in the center of the LCD panel would be affected and the cross-talk is more serious.
The existing 3D displays usually use a light guide plate with a prism structure. FIGS. 5A and 5B are perspective and planar schematic diagrams showing light trajectories of an existing light guide plate with a prism structure. A conventional light guide plate 50 has a symmetric prism structure so that light beams 52 can propagate farther parallel to the prism direction 51 and achieves more total internal reflection (T.I.R.). Light beams 53 perpendicular to the prism direction 51 propagate in a confined range and T.I.R. is difficult to achieve, thereby forming a converged light pattern. FIG. 6 shows the light patterns from a flat light guide plate (left) and a light guide plate with a prism structure (right). From FIG. 6, the degree of convergence from the light guide plate with a prism structure is greater than that of the flat light guide plate.